VOPM parses this as a 3 bit signed number, where third bit is sign bit and other is magnitude. Deflemask does something else - maybe maps values to another value, but they don't match up.

Also KS (Key Scaling) is missing as a parameter, why? It's value 0-3 affects the envelope generator decay speed and appears to be ignored. That means it's not possible to fully import all settings from a YM2151 register.I also noticed that LFO and noise settings are missing as well. It would be nice to have full access to all parameters especially in a tracker that can output VGM and playback on hardware.